What's with the different FHR ratings of these hot water tanks?

Question

We're replacing our tank with a more efficient one and we can't go tankless given the cold input temperatures in Canada and the amount of output we need (for a triplex with 7 people).

So I've been researching tanks. Both Rheem and Bradford White seem to offer the most efficient, high quality tanks. I've determined from my calculations that we could get away with something in the 100-120FHR range to serve the peak shower time in the morning for the household.

One thing I'm completely confused by though is the Bradford White high performance energy saver 25gallon tank. Every tank we've seen is around 50-75 gallons with an FHR ranging from 80-120. This 25 gallon tank says it has an FHR of 155 gallons (200 gallons FHR for the 50 gallon tank). I'm wondering first how is this possible, and second, why this listing would say it's intended for a 1 person household.

What one person would ever use 155 gallons of hot water in an hour? I'm positive that our house, with 3 showers and low flow heads at 10 minutes a shower with 90 degree rise would use about 105 gallons of hot water. What am I missing with this particular tank? It seems too good to be true that a 25gallon tank could serve us and its even more confusing to say that 155gallon FHR is only good for one person.

Any thoughts?

edit

I've realized what's wrong with my assumptions that 150FHR would serve our house. If I assume we need 105 gallons for a a worst case scenario of 3 simultaneous 10 minute showers, repeated twice (ie 6 showers over 20 minutes) then that's 105 gallons in 20 minutes, not an hour. So I need to take the recover of the tank for an hour, divide by 3 and add it to the capacity. So its recovery is 82gal/hr which means in the first 20 minutes we'd only get 45 gallons (approx) of water.

Answer 1

The power output of the water heaters is the crucial point. The "high performance" heater is 76 kBTU/Hr, while most water heaters only are around 30-40 kBTU/hour. This larger burner allowed the smaller tank to be able to "keep up with the demand" for quite a while. Another reason is because it maybe would be configured to heat to a much higher temperature, and then use the mixing valve that in includes to reduce the temperature to the reasonable level. I'd consider this "cheating" if it is what they are doing, though it is allowed by the government.

I'm guessing that the listing says one person because it is a small tank, and not based on the FHR (which they should have paid more attention to).

I don't think that you are missing anything based on the specs on FHR. But, you do need to pay attention to the size of your natural gas line. You'll need a higher flow rate with this model since it's usage rate is much higher. You also might need to provide more "leaks" in your house (and room containing the water heater) to allow for it to get sufficient air for combustion. This particular model uses air from its surroundings, instead of having a pipe to bring air in from the outside. Insufficient fresh air will generate CO (very bad for living things) and will decrease the water heater's efficiency. Make sure that you have a CO detector installed near the unit, whatever your choice ends up being.

The first hour rate is an odd metric. It measures the number of gallons that can be supplied at above 110 F, with an initial temperature of 135 F. This is very difficult to use in calculations.

I'd approach the calculations as an energy conservation problem. Use the recovery rate to calculate the burner output (1080 kJ/min in this case). Then (ignoring that entropy is gained in the mixing valve and the heat capacity and density of water depends on temperature), decide on what temperature shower you want, and what temperature water you want in the tank. Use the heat capacity of water to calculate the amount of energy stored in the water heater, the energy usage rate (power out), and the burner energy rate (power in). Assuming use of 6 gpm, input water at 7 C, the tank at 75 C, and using water at 38 C, I calculate that you would have about eight minutes of water. With the 55 gallon tank, you'd have about 18 minutes using these assumptions.

Just by looking at the 90F recovery rate, you get about 80 gallons/hour. So, you could use 80/60=1.33 gpm indefinitely (with a rise of 90 F). But, you probably only need a rise from 45 to 100 F for your shower. In this case (55 F rise), you could use about 2.2 gpm indefinitely with this particular burner.